We phoned the Maritime Police in L'Ametlla to ask what the requirements were for foreigners who live here to use boats on the sea as there seems to be lots of confusion/mis information.

The following is what we were told:

You need to obtain authorization from the Capitain Maritime.
To do this you need to take (plus copies); passport, titles (RYA/ICC/ whatever)
You need to fill in a request form and (subject to qualifications etc) they will issue the necessary documents.

The number for the office is 977 240 955
The office is in Tarragona:

Capitanías marítimas.
Oficina Principal
Arranque Rompeolas, s/n
43071 - Tarragona
España

Nigel